Method and apparatus for determining crystallization point



p 14, 1955 VAN ASSENDELFT 3,205,699

METHOD AND APPARATUS FOR DETERMINING CRYSTALLIZATION POINT Filed July31, 1962 INVENTOR. LEENDERT VAN ASSENDELFT ATTO R N EY United StatesPatent 3,295,699 METHOD AND APPARATUS FOR DETERMHNENG CRYSTALLEZATKDNPGlNT Leendert van Assendeift, Arnhem, Netherlands, assignor to AmericanEnka Corporation, Erika, N.C., a corporation of Delaware Filed July 31,1962, Ser. No. 213,751 Claims priority, application Netherlands, Aug.Ill, 1961, 268,100 8 Claims. (CI. 73-17) This invention relatesgenerally to spin bath solutions used in the viscose process and moreparticularly to a method and apparatus for continuously determining thecrystallization point of sodium sulfate in such spin baths whereby thecomponents of the bath solution can be continuously held constant.

It is known that the sodium sulfate content of spin baths used in theviscose process can be determined by density measurement. Since thedensity of the spin bath increases with the sodium sulfate content, thevalues found for the density are indicative of the sodium sulfatecontent. However, the results obtained by this measuring method are notaccurate, since the sulfuric acid and the other salts present, such aszinc sulfate and magnesium sulfate, also influence the density of thebath. Thus, fluctuations in the contents of such compounds stronglyinfluence the measuring results.

Therefore, it is an object of this invention to provide a method forcontinuously determining the sodium sulfate content in viscose spinbaths not having the disadvantages inherent in the known method.

Another object of this invention is to provide a method for accuratelydetermining the sodium sulfate content in viscose spin baths.

Still another object of this invention is to provide a method andapparatus for continuously determining the crystallization point ofsodium sulfate in viscose spin baths.

A further object of this invention is to provide a method and apparatusfor accurately determining the crystallization point of sodium sulfatein viscose spin baths which is not influenced by fluctuations in contentof other components present in the bath.

These and other objects are accomplished in accordance with thisinvention by separately and continuously determining the crystallizationpoint of the sodium sulfate in the bath and the sulfuric acid contentthereof. The sodium sulfate content is then continuously derived fromthe thus determined crystallization point and the sulfuric acid content.In carrying out the method of this invention, use is made of the factthat the sulfuric acid content, the sodium sulfate content, and thecrystallization point of the spin bath are so related that upondetermining two of these quantities, the third can then be derived.Consequently, if in a given spin bath two of these quantities are known,the third may be determined therefrom. By the method of this invention,the sodium sulfate content is derived from the sulfuric acid content,and the crystallization point, both of which are deter mined in acontinuous manner. The sodium sulfate content can then be derived fromthe values found with the aid of a graph or nomograph. However, forpractical reasons, it is preferred to employ automatic control equipmentwhich converts the data received from the apparatus used in measuringthe crystallization point and the sulfuric acid content into data whichdirectly indicates the sodium sulfate content. The latter data is thenused to control correction apparatus which maintains a constant sodiumsulfate content. Since the data received from the continuous titrationof the sulfuric acid can nearly always be used for controlling thecorrection stream of 3,205,699 Patented Sept. 14, 1965 the sulfuricacid, the spin bath entering the measuring apparatus will have aconstant sulfuric acid content. The crystallization point of the bath isthen only dependent on the sodium sulfate concentration. Thus the sodiumsulfate content may be derived directly from the crystallization point,since the sulfuric acid content always has the same value.

Other salts present in a spinning bath, such as zinc sulfate andmagnesium sulfate, have very little influence on the determination ofthe sodium sulfate content. The presence of these salts may beconsidered when interpreting the results obtained. However, theinfluence of fluctuations in the concentrations of these salts on thecrystallization point of the bath is negligible.

Conventional measuring apparatus for continuously determining thesulfuric acid content can be used, the acidity being determined bytitrating a continuous stream of liquid, for example a sodium hydroxidesolution, with a continuous stream of the bath to be examined.

The crystallization point can be determined in various ways. Inprinciple, the determination is carried out by cooling a stream of thespin bath and measuring the temperature at which crystallization occurs.For example, the spin bath is cooled and then passed through a layer ofglauber salt crystals. The cooling is so controlled that none of theglauber salt crystals are dissolved and none precipitate from the bath.Very accurate and reliable apparatus that may be used for this purpose,which continuously measures the crystallization point, comprises a fiowchannel surrounded by a cooling jacket provided with an adjustable inletfor introducing cooling liquid thereto. The flow channel opens in acrystalli ation chamber provided with discharge means. A thermometer anda sediment level indicator controlling the inlet for the cooling liquidthrough a control device are disposed within the crystallizationchamber.

In one embodiment the apparatus is so constructed that thecrystallization chamber comprises a funnelshaped, downwardly taperingvessel, the upper edge of which forms the discharge means. The flowchannel is connected at the bottom of the chamber with the sedimentlevel indicator being an immersion body freely adapted to move both inan upward and downward direction. The immersion body used is of a typethat will sink in the liquid being examined but will float on the layerof crystals.

In such an apparatus the spin bath liquid enters the flow channel, whereit is cooled until the sodium sulfate crystallizes out of solution, thecrystals forming a layer in the bottom of the crystallization chamber.As the layer of crystals increases, the immersion body is raised so thatthe control device connected therewith causes the inlet opening for thecooling liquid to constrict. As a result, the spin bath stream is cooledto a less degree. This continues until the liquid passing through thelayer of sodium sulfate crystals is no longer saturated so that some ofthe sodium sulfate goes back into solution. The immersion body will thendescend, thus slightly enlarging the inlet opening for the coolingliquid, as a result of which the spin bath stream is cooled againslightly. Consequently, the amount of cooling liquid is controlled bythe immersion body so that a solution which is just saturated withsodium sulfate continuously enters the crystallization chamber at atemperature close to the crystallization point.

The temperature is read from the thermometer present in the chamber andis a measure of the amount of sodium sulfate in the spin bath. By meansof this reading, the apparatus controlling the sodium sulfate content inthe bath may be adjusted. In practice, a so-called control thermometeris used which regulates the apparatus correcting the sodium sulfatecontent through a conventional the flow channel being connected to onepart, and the discharge member to the other. In such an apparatus, thelayer of sodium sulfate crystals forms on the partition. The sedimentlevel indicator of this apparatus is preferably a pressure meter whichmeasures the pressure drop in the liquid through the layer of crystals.The pressure drop increases as the layer of crystals becomes thicker. Itis also possible to use a combination of two light sources and twophoto-electric cells such that the layer of crystals forms between thelight sources and the photo-electric cells. They are arranged so thatone photo cell receives light while the other does not when the layer ofcrystals has the correct depth. When the depth of the layer increases,the beam of light received by one photo-electric cell is interrupted, asa result of which the stream of cooling liquid is reduced. When thethickness of the layer decreases, the cell which first received no lightnow receives light, as a result of which the stream of cooling liquid isincreased.

The crystallization point can also be determined through use of the heatreleased upon crystallization or the sudden drop'in electricalconductivity upon crystallization. However, the simplest apparatus isthat in which the cooling of the bath stream is so controlled that thebath is in equilibrium with the sodium sulfate crystals.

For purposes of illustration, there are shown in the drawings severalembodiments of the apparatus according to this invention, However, theinvention is not intended to be limited to the precise instrumentalitiesand arrangements shown.

FIGURES 1, 2, and 3 are vertical views in section showing threeembodiments of the apparatus for cooling a spin bath stream down to itscrystallization point. The cooling is then adjusted by a control devicewhich is regulated by apparatus measuring the depth of sodium sulfatecrystals through which the cooled spin bath stream passes.

In FIGURE 1 the apparatus for the determination of the crystallizationpoint designated generally at 1 comprises a vertical conduit 2surrounded by a cooling jacket 3 provided with a supply line 4 and acontrol valve 5. Cooling liquid flows into cooling jacket 3 throughsupply line 4 and is discharged through line 6.

Vertical conduit 2 opens in the lower end of funnelshaped vessel 7surrounded by a second funnel-shaped outer vessel 8. In vessel .7 a.thermometer 9 and an immersion type sediment level indicator 10 arepositioned. Sediment level indicator 10 is connected by line 11 withcontrol device 12 which adjusts the opening of control valve 5. Adischarge line 13 connected at the base of outer vessel 8 opens intocollector 14.

In using the apparatus, sodium sulfate crystals are placed in vessel 7.The liquid spin bath enters the apparatus at the lower end of conduit 2.In conduit 2 the bath is cooled down to its crystallization point Thefeed rate of the spin bath is controlled so that the force of gravitydoes not cause the crystals in the vessel 7 to enter conduit 2. When theamount of cooling liquid entering cooling jacket 3, by way of the line4, is such that the liquid spin bath on entering vessel 7 has assumedthe temperature of the crystallization point, no sodium sulfate willdissolve from the crystal layer or precipitate from the bath. However,if the temperature of the spin bath is Somewhat higher than thecrystallization point, sodium .4 sulfate will go into solution. As aresult, sediment level indicator 10 will move downward, thus increasing,by way of the control device, the opening of control valve 5. The amountof cooling liquid entering the cooling jacket 3 will increase, so thatthe temperature of the spin bath entering the vessel 7 will fall. Whenthetemperature of the spin bath falls below the crystallization point,sodium sulfate will precipitate from the spin bath, so that the layer ofcrystals present in vessel 7 will increase, thus resulting in thecontrol valve opening being reduced. The temperature of the spin bath atthe thermometer 9 deviates only slightly from the crystallization point.The spin bath leaves the vessel 7 by Way of the space between the vessel7 and the outer vessel 8 through the acid discharge line 13, thuspreventing the occurrence of undesirable variations in temperature inthe vessel 7 due to the exchange of heat with the surrounding apparatus.

The reading on thermometer 9, in combination with the sulfuric acidcontent, which is determined conventionally, is a measure of the sodiumsulfate concentration. This concentration may be determined, forexample, with the aid of a graph or a nomograph.

In FIGURE 2, numerals 1, 2, 3, 4, 5, 6, 9, 11, 12, and 14 refer to partshaving the same function as in FIG- URE 1. Near outlet 15 the conduit 2is divided into two parts by a piece of fiber glass fabric 16, Outlet 15ends in a vessel 17 which is provided with an overflow 18. Above thepiece of fiber glass fabric 16 there is a pressure meter 19 which isconnected with control valve 5 by way of the line 11 and the controldevice 12. Below the fabric 16 is positioned a thermometer 9.

When using the apparatus, an increase or decrease in the amount ofsodium sulfate crystals results in an increase or decrease in pressurein the liquid over the fabric 16. These variations in pressure arecommunicated to the control valve 5 by way of the pressure meter 19 andcontrol device 12. Thermometer 9 always indicates a temperature whichdeviates only slightly from the crystallization point of the bathliquid.

In FIGURE 3, numerals 1, 2, 3, 4, 5, 6, 9, 12, and 14 refer to partshaving the same function as in FIGURES 1 and 2. Section 20 of conduit 2is of glass. A partition 21 and fiber glass fabric is positioned insection 20 on which a layer of crystals may precipitate. The height ofthis crystal layer is measured by photo cells 22 and 23 which areactivated by light source 24. Photo cells 22 and 23 are connected withcontrol device 12 so that said control device increases the opening ofcontrol valve 5 when photo cell 23 receives light as a result of adecrease in depth of the crystal layer, and narrows the opening whenphoto cell 22 receives no light as a result of an increase in depth ofthe crystal layer.

Using the method of this invention, it is now possible to accuratelydetermine the sodium sulfate content of a circulating spin bath and keepthe sodium sulfate content constant within very narrow limits by meansof the measured values. As a result, spinning, conditions can be keptmore constant than heretofore thought possible. Not only will this leadto a more uniform yarn, but use may be made of spinning processes whichare very critical regarding the sodium sulfate concentrations withoutthere being any risk of inadmissable variations in this concentration.If the sodium sulfate content is automatically controlled and at thesame time the sulfuric acid content is controlled, an additionaladvantage results in that the zinc sulfate content may be determined ina simple manner by density measurement, since the only remaining factorcontrolling the density of the spin bath is the zinc sulfate, providedthere are no other salts present that would affect the density. Thus bycontinuously measuring the density in a conventional manner and usingthe measuring apparatus of this invention, it is possible toautomatically control the sulfuric acid, sodium sulfate, and zincsulfate contents very accurate and in a continuous manner.

The invention is further illustrated by the following example.

Example The crystallization points of four different spin baths, a, b,c, and d were determined with the apparatus shown in FIGURE 1. By meansof titration the sulfuric acid content was also determined. From thevalues obtained the sodium sulfate content was determined with the aidof a graph. The density of the liquids and the sodium sulfate and zincsulfate contents were then determined by conventional methods known inchemical analysis. The values obtained are summarized in the tablebelow.

The table clearly shows that it is possible in accordance with theinvention to accurately determine the sodium sulfate content and that avarying zinc sulfate content does not influence the results. With themethod of the prior art in which sodium sulfate content is determined bydensity measurements, variations in zinc sulfate content influence theresults considerably, since zinc sulfate contributes to the density ofthe spin bath in the same magnitude as sodium sulfate.

While the invention has been described in connection with the foregoingdrawings and examples, many changes, embodiments, and modificationswithin the scope of the invention will be apparent to those skilled inthe art. Accordingly, the invention is intended to be limited only asset forth in the following claims.

What is claimed is:

1. Apparatus for continuously determining the crystallization point ofcirculating solution comprising a conduit for carrying said solution, acooling jacket surrounding said conduit provided with liquid inlet andoutlet means, said inlet means being adjustable for controlling the fiowof cooling liquid into said cooling jacket, a crystallization chamberconnected to said conduit for receiving solution therefrom, solutiondischarge means connected with said chamber, a sediment level indicatorpositioned in said chamber for indicating a surface level of crystalsdeposted therein, means connected to said sediment level indicator foradjusting said inlet means to maintain said level substantiallyconstant, and a thermometer positioned Within said chamber forcontinuously determining the crystallization point of said solution.

2. The apparatus of claim 1 in which the crystallization chambercomprises a funnel shaped vessel connected with said conduit at the apexof said funnel-shaped vessel.

3. The apparatus of claim 1 in which the crystallization chambercontains a partition permeable to the solution but impermeable tocrystals whereby crystals collect on said partition and the solutionpasses therethrough.

4. The apparatus of claim 1 in which the sediment level indicator is animmersion body adapted to move freely in a vertical direction.

5. The apparatus of claim 1 in which the sediment level indicator is apressure meter.

6. Apparatus of claim 1 in which the sediment level indicator comprisestwo light sources and two photoelectric cells adapted to respondaccording to the crystal depth.

7. Apparatus for continuously determining the crystallization point of acirculating spin bath solution used in the viscose process comprising aconduit for carrying a continuous stream of the solution, a coolingjacket surrounding said conduit provided with liquid inlet and outletmeans, said inlet means being adjustable for controlling the flow ofcooling liquid into said cooling jacket, a funnel shaped crystallizationchamber containing sodium sulfate crystals to an initial depth connectedwith said conduit at the apex of said funnel and provided with solutiondischarge means, a sediment level indicator positioned in said chamberfor sensing any variation in the initial depth of crystals collectedtherein, means connected with said sediment level indicator forcontrolling the inlet of said cooling jacket whereby the cooling of saidspin bath is continuously regulated according to the crystal depth insaid chamber, and means within said chamber for measuring thetemperature of the solution passing therethrough whereby thecrystallization point of said solution is continuously determined.

8. A method for continuously determining the crystallization point of acirculating spinbath used in the viscose process, comprising the stepsof (a) cooling a stream of spinbath to a point where crystallizationjust starts to occur,

(b) passing the stream of cooled spinbath through a layer of sodiumsulfate crystals of known thickness,

(0) detecting any changes in thickness of the layer of the crystals,

(d) varying the amount of cooling of the stream in response to saidchanges to maintain a substantially constant thickness in the layer ofsodium sulfate crystals, and

(e) measuring the temperature of the spinbath at a point adjacent thelayer of sodium sulfate crystals.

References Cited by the Examiner UNITED STATES PATENTS 3,026,710 3/62Lupfer 7317 3,031,880 5/62 Findlay 7317 3,060,318 10/62 Ouvrard 73l7 XLOUIS R. PRINCE, Primary Examiner.

JOSEPH P. STRIZAK, RICHARD C. QUEISSER,

Examiners.

1. APPARATUS FOR CONTINUOUSLY DETERMINING THE CRYSTALLIZATIN POINT OFCIRCULATING SOLUTION OCMPRISING A CONDUIT FOR CARRYING SAID SOLUTIN, ACOOLING JACKET SURROUNDING SAID CONDUIT PROVIDED WITH LIQUID INLET ANDOUTLET MEANS, SAID INLET MEANS BEING ADJUSTABLE FOR CON ROOLLING THEFLOW OF COOLING LIQUID INTO SIAD COOLING JACKET, RECEIVING SOLUTIONTHEREFORM, SOLUTION DISCHARGE MEANS CONNECTED WITH SAID CHAMBER, ASEDIMENT LEVEL INDICATOR POSITIONED IN SAID CHAMBER FOR INDICATION ASURFACE LEVEL OF CYRSTALS DEPOSTED THEREIN, MEANS CONNECTED TO SAIDSEDIMENT LEVEL INDICATOR FOR ADJUSTING AID INLET MEANS TO MAINTAIN SAIDLEVEL SUSTANTIALY CONSTANT, AND A THERMOMETER POSITIONED WITHIN SAIDCHAMBER FOR CONTINUOUSLY DETERMINING THE CRYSTALLIZATION POINT OF SAIDSOLUTION.